ok small disclaimer: I'm not quite sure if I read all the replies here so I hope I won't repeat previous points.
(it kinda got out of hand, I apologise for the length.)
First of all, small story time: I'm myself am bisexual but literally didn't know this fact about myself (even though there were PLENTY of hints) until someone literally asked me in highschool if I had a crush on a boy or a girl. I never grew up in a homophobic region, my parents & families are really liberal and open and I've even seen gay and lesbian couples on TV before. I just never got the idea that my weird obsession with boobs might be a sign that I ... also like girls.
AKA: your character doesn't need to be blatantly homophobic or come from a homophobic background to consider themself straight at first! I wish more people would just include the "totally oblivious" type ("what do you mean you don't appreaciate your best friends' buttocks like that?") instead of internalised homophobia because THAT is a trope played to death (and needs to be handled with a lot of care. If you are just starting out with getting into the whole topic, I'd actually advise against it.)
Also, for people who currently are discussing about "yeah but can that one person turn you gay/straight?" - uhhh... in most cases, if that happens, the person attracted to them usually defines themself as bi or pan then (of course would depend on the time period but if we go by today's standard, they would.) They just aren't as homo- or heterosexual as they innicially thought, that's all about that. How open and "out of the closet" they are about it is a completely different story.
The thing about fluid sexuality: it's that thing where you first thought "well, I'm straight!" - might be because that's just what everyone told you to be, or you just experienced more attraction towards one gender. Then later you might realise "well, maybe I like girls/boys too!" and identify yourself as bisexual. Then you decide "well actually, I really don't like guys? sex with them always felt very uncomfortable, maybe I'm actually a lesbian" until you might realise "wait, I have no sexual attraction to anyone whatsoever, I'm actually asexual but can fall in love with every gender!" boom. Asexual panromantic.
When people say "sexuality is fluid" it means that you are constantly discovering things about yourself and only because you once labelled yourself in one way, you might realise that this label doesn't apply anymore. Also, trauma can change your perception of sex or other things, so this might also influence the perception of your identity until you later find out "wait, that was just my trauma talking!"
People say it because oftentimes, if people need to relabel themselves 5 times, others easily get sceptical, even though it's a very valid thing to relabel yourself (some also just go with "I'm queer" if they are still very unsure about anything but at least can they aren't straight to forego the endless debates.)
It also kinda tackles the whole bisexual experience where you sometimes are more attracted to one gender but later more towards the other. It's just how it is.
However, it doesn't mean that a lesbian who knew she was only into girls since she was 5 and continued to only be into girls and now is 80 can be "turned" into liking men, I'm pretty sure that lady already knows what she's about with her wife on a farm and 20 cats.
Same goes for straight people who even tried "to be gay" and just realised they aren't attracted to the same gender at all.
I personally don't like the "everyone is a little bisexual" because it's just not true - there are just more bisexuals who only have a minor attraction towards another gender than they are majorly attracted to.
But now to the actual question!
If you say
can you sepcify more? Because homophobia wasn't always the same level of bad throughout all of history, it also depends a lot on which country and region you're looking at. It wasn't always illegal, there oftentimes were loopholes in the law and queer people rather feared the social downfall rather than jail (18th century was actually pretty lax as a lot of aristocrats had orgies so it was a bit difficult to keep track, and Edo Period Japan had a similar system of homosexual relationships with their apprentices as the ancient Greeks had.) There were many codes that helped identify each other (I think the most infamous is the "hankerchief") and it also depended on your social status whether you would be shunned by others or not (see Lord Byron). If you have powerful friends, you might get away with much more than if you don't have them.
This reads just Yikes to me. Currently I hope it is about werewolves because having any human race "go feral" at a certain age is just... uhhhhhhh.........
With this premise tbh, I doubt that the gay character would even try anything anymore. Remember that he got labelled a "monster" and got slurs thrown around his head in addition to deeply rooted mistrust from the "straight" MC. If anything like this happened to one of my queer friends (or literally anyone I know) I'd say run and don't look back, they are not worth your time and effort. I seriously couldn't think of a reason why I'd want to be in a relationship with such a person.
BUT since this is only about how MC might actually fall in love with the gay character instead of them getting together, here we go!
Supposedly straight MC could notice over time that he deeply misses the gay character but doesn't know why. Some people simply know they're in love only if their love isn't available anymore. Sprinkle in some daoubt and guilt, wishing he'd come back, all the secret pining yadda yadda. He might think about the confession and "how can a man love another man" and slowly overthink his homophobic ideology.
Maybe he tries to learn more about the gay character in some way (though this can easily tread in stalker behaviour) and thus understand him more, adjust more to his thinking etc. But seriously, have the gay character keep his distance, that's a step MC has to do himself without him (plus gay character shouldn't need to go about educating someone who mistreated him so badly.) Also note that this most likely will take years for the gay character to heal and for the supposedly straight character to grow out of his bullshit.
Then, years later, MC might try to contact the gay character to reconcile but tbh he must earn gay chara's trust first before anything can come out of it, and that might take ...again some time.
(I just read it now, basically all what Poppy mentioned.)
You should also note that a lot of kids shows' villains are queer-coded, and having a queer character be called a monster (plus the link with being "feral") can get you into a VERY tight spot. Please please PLEASE do thourough research on not only the history of the queer community in the region you're dealing with but also racial and queer stereotyping and how to avoid/undermine them.